Referring to FIG. 1, a conventional backlight module 1 includes a light guide plate 11, a light source 12 disposed adjacent to one side of the light guide plate 11, and a light reflection sheet 13 disposed adjacent to another side of the light guide plate 11. The light guide plate 11 has a light incidence surface 111 that faces and is spaced apart from the light source 12, a lateral surface 112 that is opposite to the light incidence surface 111, a light reflection surface 113 that interconnects the light incidence surface 111 and the lateral surface 112 and faces the light reflection sheet 13, a light exit surface 114 that is opposite to the light reflection surface 113 and interconnects the light incidence surface 111 and the lateral surface 112, and a plurality of microdots 115 that are disposed on the light reflection surface 113.
According to Snell's law, the light emitted from the light source 12, after entering the light guide plate 11 through the light incidence surface 111 thereof, is supposed to undergo total reflection in the light guide plate 11 since the refraction index of the light guide plate 11 is larger than that of air. Nevertheless, the microdots 115 are able to prevent total reflection of light by directing the light in the light guide plate 11 out of the light guide plate 11. The light directed out of the light guide plate 11 is reflected by the light reflection sheet 13 into the light guide plate 11, thereby being emitted out of the light guide plate 11 through the light exit surface 114 to serve as useful light of the backlight module 1.
Generally, light paths in the backlight module 1 can be divided into the following three categories. First, a light beam 121 emitted from the light source 12 at a relatively large emission angle undergoes more times of total reflection after entering the light guide plate 11, and hence easily reaches one of the microdots 115 when traveling still adjacent to the light incidence surface 111 of the light guide plate 11. Secondly, a light beam 122 emitted from the light source 12 at a relatively small emission angle, after entering the light guide plate 11, normally reaches one of the microdots 115 when traveling only away from to the light incidence surface 111 of the light guide plate 11. Thirdly, a light beam 123 emitted from the light source 12 and entering the light incidence surface 111 of the light guide plate 11 at an angle of nearly 90 degrees undergoes mush less times of total reflection compared to the light beams 121, 122, thereby being usually emitted out of the light guide plate 11 through the lateral surface 112 without reaching the microdots 115. Based on these non-uniform light paths, the light emission efficiency is unsatisfactory.
Referring to FIG. 2, each of the conventional microdots 115 often has a plano-convex structure which has a bowl shape or a bowl-like shape, thereby having a curve 116. Therefore, the light reflected by the curve 116 to the light exit surface 114 is of a diffusion type, and hence can be hardly concentrated. Furthermore, since the reflection angle at which light is reflected to the light exit surface 114 by the curve 116 of each of the conventional microdots 115 cannot be adjusted, the emission angle at which light is emitted from the light exit surface 114 can hardly be adjusted, and thus the light utilization rate cannot be improved. In addition, due to the poor light concentration attributed to the conventional microdots 115, the directivity of the light emitted from the light exit surface 114 is unsatisfactory. Accordingly, even if the light guide plate 11 is used in combination with a turning prism sheet, the light emission efficiency can hardly be enhanced sufficiently.